1. Field of the Invention
This invention relates to digital-to-analog converters. More particularly, this invention relates to such converters provided in integrated-circuit (IC) format on monolithic chips.
2. Description of the Prior Art
Digital-to-analog converters generally have comprised a plurality of switches which are selectively operated by an input digital signal to produce corresponding binarily-weighted contributions to an analog output signal. For solid-state converters, it was found that current switches were superior to voltage switches, and an excellent example of such a design, using discrete elements, is shown in U.S. Pat. No. 3,685,045. That patent also discloses the important concept of providing a matched reference transistor, in combination with means for automatically adjusting the power supply voltage so as to maintain the reference transistor current constant, thereby also maintaining the switch currents constant.
Considerable effort has been devoted to applying integrated-circuit techniques to digial-to-analog converters. However, difficult problems have been encountered, particularly in converters designed to handle relatively large digital numbers, e.g. upwards of 8 bits. One significant advance in that regard (see U.S. Pat. No. 3,747,088) was to divide the switches into separate but identical groups, and to provide attenuation means to reduce the current contributions of the groups representing lower-order bit. For example, a 12-bit converter can be formed of three separate IC switch modules each containing four switches (such modules now being commonly referred to as "quad switches"). The latter U.S. Pat. No. 3,747,088 also teaches the highly advantageous concept of binarily scaling the areas of the emitters of the constant-current transistors, so as to provide uniform current density within the conductive regions of the transistors, thereby minimizing any differential variations in V.sub.BE of the current switches.
Notwithstanding such developments in the design of solid-state digital-to-analog converters, there still has existed a need for improved integrated-circuit converters capable of handling relatively large digital numbers. It particularly has been desired to provide improved operational characteristics, e.g. accuracy and speed. Also, there has been a need for IC converters capable of performing a multiplier function with accuracy. And such improved performance converters particularly should be capable of being manufactured at reasonable cost, using straightforward IC processing techniques.